The invention relates to a procedure for testing the function of a lamp circuit, consisting of at least one lamp, by measuring the current and voltage.
The nominal power of a lamp is the electrical power input of the lamp when defined standard conditions are present, in particular the application of a nominal voltage; it is given by lamp manufacturers as a lamp parameter alongside the nominal voltage, and is required by manufacturers of lighting systems as a standard value to be maintained. However, some lamps deviate significantly from the given standard values, which may lead to faults or defects in the lighting control or the lamps.
Lighting systems are also used in safety-related applications, in particular in motor vehicles. With safety-critical applications of this type, it is necessary to check during operation for defects, or for non standard-compliant lamps which may have been installed.
For example, when filament lamps are triggered in the motor vehicle, the switch output can already be diagnosed by the electronic system, for example, and information can be provided regarding the state of the load. By recognising the operating states and/or the measurement of the determining electrical values, the failure of the lamp can be detected, and a message can be sent to the driver or to a diagnosis system.
The precision of the diagnosis procedure is restricted by different parameters, such as the precision of the measurements, for example, and above all by the electrical model of the lamps used.
The simplest procedure for determining the lamp state is a digital determination of the output voltage when in a switched-off state. When the lamp is defective, the circuit is interrupted, which can be detected on the voltage level.
Improved procedures use a current measurement when the lamp is in a switched-on state, with a measuring resistor or current mirror switches or integrated solutions such as the so-called “SenseFETs” with a control input and a current signal output, as illustrated in FIG. 1.
However, the voltage on the lamp circuit cannot be adopted accordingly as a constant value of the nominal voltage in a wide range of applications, in particular in motor vehicles. However, if the voltage deviates from the nominal voltage, the current power input produced from the voltage and current does not correspond to the nominal power.
In addition, the current lamp resistance may fluctuate due to:                the use of different lamp types        the different nominal voltage of the different lamp types        deviations from different manufacturers        scattering within a lamp type        lamp ageing.        
Particularly problematic is the diagnosis with two or more lamps which are switched in parallel, which can only be detected, if at all, when the electronic system is calibrated.
In addition, the use of intact lamps which deviate from the specification, however, or a fault in individual lamps, can lead to faults on the control unit or other lamps which are switched in parallel with the lamp.
A procedure for testing the function of a lamp circuit consisting of at least one lamp is known from U.S. Pat. No. 5,578,998. The function test is conducted by measuring the current and the voltage on a resistance.
A further procedure for testing the function of a lamp is known from EP-A-0 507. In order to test the function, the current and the voltage are measured on a resistance.